Links, including lower links, are used in tractors as components of three point hitches, in order to fasten implements to the tractor, such as for example manure spreaders, plows and other ground working implements. To facilitate the attachment of the implement to the tractor, two-part lower links are known which are constructed from two struts which are able to be telescoped relative to one another and separated from one another. The struts are provided, at one end, with ball joint eyes by which they may be fastened to associated points of the tractor and the implement. The struts are thus pushed into one another by reversing the tractor and when reaching a desired end position are fixed to one another by a locking device, so that the implement is then fixed to the tractor ready for operation. The locking device may be released again on completion of the operation.
Such a typical lower link is disclosed in U.S. Pat. No. 5,327,978. A first strut, which may be connected to the tractor, has a channel to the rear into which a second strut which may be connected to the implement may be inserted. In the region of the second strut, a saw tooth-like recess is incorporated. The first strut comprises in the region of the channel a pocket in which a locking plate is pivotably coupled by means of two coaxial pivot pins attached on both sides, which extend through bores in the locking plate and in the side wall of the pocket. The locking plate is pretensioned by means of a spiral spring, which extends between the outer face of the locking plate and the inner wall of the pocket, toward the channel. As a result, the locking plate is pressed into the recess by means of the spiral spring and locks the struts to one another. A knob is fastened to the locking plate at the side, which also extends through a slot in the side wall of the pocket and allows manual unlocking, by an operator gripping the knob and moving the locking plate therewith counter to the pretensioning of the spring, out of the recess of the second strut. In similar embodiments of this lower link currently on the market, the spiral spring comprises a spring coil arranged coaxially to the pivot pin and two limbs, of which one bears against the locking plate and the other bears against the inner wall of the pocket.
In this arrangement, it is regarded as a drawback that the mounting of the locking mechanism is relatively complicated, as the locking plate has to be moved by the spiral spring through the channel into the interior of the pocket and fixed there by inserting the pins.
U.S. Pat. No. 2,717,163 discloses a coupling arrangement for connecting a tractor to an attachment or trailer which also comprises two telescopic struts, which may be fastened to one another by a locking arrangement. A locking plate cooperating with a projecting shoulder of the second strut is arranged inside a housing connected to the first strut. The end of the locking plate facing the second strut is pivotably attached between the upper face and the rear face of the housing and a pin arranged thereunder in a U-shaped pocket. The other end of the locking plate is provided with a blind threaded portion, into which a screw is screwed, and which extends upwardly through a bore in the housing. A leaf spring is riveted to the housing at one end, and at the other end is connected to the screw head and pretensions the locking plate downwardly, i.e. toward the second strut.
In this case, it is regarded as a drawback that the spring is located outside the housing and accordingly is unprotected against soiling. The attachment of the spring is also very time-consuming due to the rivets and the connection to the screw head.